Neurons

What are the key sensory systems?

Sight

Touch

Smell

Hearing

What are the three types of neurones?

Sensory Neurones

Motor Neurones

Relay Neurones

Draw the structure of a sensory neuron with labels

Draw the structure of a motor neuron with labels

Draw a relay neuron with labels

What does a sensory neurone do?

Transport electrical impulse from receptors to CNC

What does a motor neurone do?

Transport an electrical impulse from the CNC to an effector

What does a relay neurone do?

Process signals from multiple receptors

What components make up; the CNS (Central Nervous System)

Brain and Spinal Cord

What components make up the PNS (Peripherary Nervous system)

Nerves around the body

What are the key proteins in nerve impulses?

Na⁺ / K⁺ Pumps

Voltage gated K⁺ Channel Proteins

Voltage gated Na⁺ Channel Proteins

Sensory Receptor Na⁺ Channel Proteins

What is the value of resting potential?

-70mV

What happens at resting potential?

The Na⁺ And K⁺ Pump actively transports Na⁺ And K⁺ through the membrane in a 3:2 ratio to maintain -70mV

What ratio are Na and K ions pumped through the membrane

3:2

What happens during a generator potential?

A stimulus opens the gate to the Na⁺ Sensory Receptor Proteins

This allows Na⁺ to move in via facilitated diffusion, increasing the charge to -55mV

This starts to depolarise the membrane

Why is it important that -55mV is reached during the generator potential?

If this charge is not reached, the gates shut, and nerve returns to resting potential at -70mV

What value is an action potential?

+40mV

What happens to generate an action potential?

Na⁺ Voltage Gated channel Proteins open at -55mV, causing more Na⁺ to diffuse through

This is a positive feedback process as it increases depolarisation

40mV is reached

What happens during repolarisation?

K⁺ Voltage gates open at 40mV and Na⁺ close, causing K⁺ To flood out of the membrane, drastically decreasing the potential difference to below 70mV.

This is hyper-polarisation

Once the potential difference reaches below 70mV, the K⁺ voltage gated channel proteins shut, allowing the membrane to stabilise to 70mV.

Label this graph with the labels:

Resting Potential

Generation Potential

Action Potential

Hyperpolarisation

What is a nerve impulse?

Transmission of an action potential along an action

What is the other name for a nerve impulse?

Wave of depolarisation

What is the movement of an action potential known as?

A local circuit

Where do the Na⁺ Ions travel to, and how>

They move towards the negative region in the axon down a concentration gradient

What happens when the threshold potential is reached, in terms of Na⁺ Movement

The ions will diffuse along the axon to the next resting region once the action potential is generated

What is the refractory period?

This is the time when a new action potential cannot pass along the neurone

Why is there a repolarisation period? [3]

• Ions must be redistributed to restore resting potential

• Sodium voltage gated channels close

• Another impulse cannot be generated

Why are refractory periods important? [3]

• Ensures impulses are separated

• This determines the maximum frequency of impulse transmission

• Impulse therefore only passes in one direction, along the axon

What is the ‘all or nothing principal’

If threshold potential is reached, an action potential must occur. If threshold potential is not reached, nothing happens.

Why is the ‘all or nothing’ principal important?

It shows that action potentials are always the same size, and a large stimuli will increase the frequency, not the size, of the impulses.

What factors affect speed of conduction? [3]

• Axon diameter

• Temperature

• Myelination

How does axon diameter affect speed of conduction, and why? [2]

• An increase in diameter increases the speed

• This is due to the lower resistance to flow of ions in the cytoplasm

How and why does temperature affect the speed of conduction? [4]

• An increase in temperature increases the speed of conduction

• This is due to ions diffusing faster

• >40°C, conduction slows down

• This is due to the denaturing of the transport proteins

How does myelination affect the speed of conduction, and why dopes this change it? [4]

• Myelination means depolarisation can only occur at the nodes of ranvier

• This is due to the fact myelin is not permeable to ions

• Since action potential only occurs at the nodes of ranvier, the potential ‘jumps’ across the nodes

• This is called saltory conduction

What are the advantages of saltory conduction? [2]

• Longer local circuit

• More energy efficient

Describe the basic structure of a pacinian corpuscle [3]

• Single nerve fibre surrounded by layers of connective tissue which are separated by viscous gel and contained by a capsule

• Stretch mediated Na⁺ Channels on plasma membrane

• Capillary runs along base of tissue

How does the pacinian corpuscle respond to pressure? [3]

• Pressure deforms the membrane, causing stretch-mediated Na⁺ Channels to open

• If influx of Na⁺ raises membrane to threshold potential, a generator potential is produced

• Action potential moves along sensory neuron.

What are the additional features of a myelinated neuron? [3]

• Schwann Cells

• Myelin Sheath

• Nodes of Ranvier

Named three processes Schwann Cells are involved in.

• Electrical insulation

• Phagocytosis

• Nerve Regeneration

Where are myelinated neurones found in the body?

Most neurons in the central and peripheral nervous systems

Where are non-myelinated neurons found?

Group C Nerve fibres involved in transmitting secondary pain

What is the function of a synapse? [4]

• Ensures action potentials only travel one way

• Neurotransmitters send impulses between neurons/from neurons to effectors for excitatory or inhibitory response

• Summation of sub-threshold impulses

• Convergence/Divergence

What happens in the post-synaptic neuron when an action potential is transmitted between neurons

1. Neurotransmitters binds to specific receptor on postsynaptic membrane

2. Ligand-gated Na⁺ ions open

3. If influx raises membrane threshold potential, action potential is generated

What happens in an inhibitory synapse?

1. Neurotransmitters binds to and opens Cl⁻ channels on postsynaptic membrane and triggers K⁺ channels to open

2. Cl⁻ moves in & K⁺ moves out via facilitated diffusion

3. P.d becomes more negative: Hyperpolarisation occurs so no a.p

Define summation

Neurotransmitters from several sub-threshold impulses accumulates to generate action potential

What are the two types of summation

Temporal and Spatial

What is the difference between temporal and spatial summation?

Temporal: one presynaptic neuron releases neutransmitter several times in quick succession

Spatial: several presynaptic neurons release neurotransmitter at once

What happens at a synapse once an action potential arrives at the synaptic terminal? [4]

1. Voltage gated calcium ion channels open

2. There is a rapid influx of calcium ions into the presynaptic cell

3. Vesicles containing ACh fuse with the presynaptic membrane

4. ACh is released and diffuses across the synaptic cleft

What happens once ACh leaves the presynaptic neurone [4]

1. ACh binds with receptor proteins in the post synaptic membrane

2. ACh binding causes the opening of Na⁺ ion channels within the receptor proteins

3. There is an influx of Na⁺ into the post synaptic cell- initially at a relatively low rate

4. The post synaptic cell starts to become depolarised

What happens if the depolarisation of the post-synaptic cell reaches threshold potential? [3]

1. Voltage gated Na⁺ channels open

2. This causes a rapid influx of Na⁺ into the postsynaptic cell

3. This depolarises the postsynaptic cell even more, generating an action potential

How is ACh recycled? [4]

1. Acetylcholinesterase breaks down ACh into choline and acetate

2. Choline is transported back into the presynaptic cell

3. Choline combines with Acetylcholine CoA forming ACh, which is repackaged into vesicles

4. This ACh resynthesis requires ATP from mitochondria in synaptic terminal

Define convergence and divergence

Convergence: Multiple inputs, one output

Divergence:One input, multiple outputs